1. Field of the Invention
This invention relates to a heating assembly and an image-forming apparatus, a process for producing a silicone rubber sponge, and a process for producing a silicone rubber sponge roller usable as a press roller for an image-heating assembly of electrophotographic image-forming apparatus such as copying machines and laser beam printers.
2. Related Background Art
Heating assemblies are conventionally in wide use as, e.g., heat-fixing assemblies for fixing unfixed images onto recording mediums used in image-forming apparatus, image-heating assemblies for heating recording mediums to modify their surface properties such as gloss, and heat-treating assemblies for drying or laminating target materials by heating.
A prior-art heating assembly will be described below taking the case of a heat-fixing assembly equipped in image-forming apparatus such as electrophotographic copying machines and printers.
The heat-fixing assembly of image-forming apparatus is a unit by which an unfixed image (toner image) corresponding to intended image information, formed and carried on a recording medium (such as transfer sheet, electrostatic recording paper, electrofax paper and printing paper) by a transfer system or direct system is thermally fixed as a permanent fixed image onto the surface of the recording medium. Widely used as the heat-fixing assembly is an assembly of contact heating type in which a heating means and a pressing means are face to face brought into pressure contact to form a pressure contact nip (a fixing nip), and a recording medium to which images are to be fixed is guided into the pressure contact nip and is held and transported between them to cause the unfixed image to fix to the recording medium surface by heat and pressure, as in a heating roller system and a film heating system. These heating systems are described below.
A) Heating Roller System
This is basically constituted of a pair of rollers pressure-contacted in parallel, consisting of a heating roller (fixing roller) as the heating means and an elastic press roller as the pressing means. The pair of rollers are rotated and the recording medium to which images are to be fixed is guided into a pressure contact nip between the pair of rollers and is held and transported between them to cause the unfixed image to fix to the recording medium surface by heat and pressure, i.e., by the heat of the heating roller and the pressure at the pressure contact nip.
B) Film Heating System
The film heating system is disclosed in, e.g., Japanese Patent Applications Laid-Open No. 63-313182, No. 2-157878, No. 4-44075, No. 4-44083, No. 4-204980 and No. 4-204984. The film heating system has a heating element and a heat-resistant film (fixing film) as the fixing means and has an elastic press roller as the pressing means. The heat-resistant film is brought into pressure contact with the heating element by the aid of the elastic press roller to form a pressure contact nip, and the heat-resistant film is brought into close contact with the heating element and is transported being slidably rubbed, where a recording medium to which images are to be fixed is guided between the heat-resistant film and the elastic press roller at the pressure contact nip to allow this recording medium to be transported together with the heat-resistant film. At this stage, the unfixed image is fixed to the recording medium surface by heat and pressure, i.e., by the heat applied from the heating element to the recording medium through the heat-resistant film and by the pressure applied at the pressure contact nip. The recording medium is separated from the heat-resistant film after it has passed the pressure contact nip.
In the heating assembly of this film heating system, a linear heating element with a low heat capacity can be used and a thin film with a low heat capacity as the heat-resistant film, and hence energy can be more saved and the waiting time can be more shortened (can be started more quickly). Also, the heating assembly of this film heating system has types of a system in which an endless belt is used as the heat-resistant film and, as a means for driving the film rotatingly, a drive roller is provided on the inner peripheral side of the film to drive the film rotatingly while a tension is applied to the film, and a system in which the film is externally loosely fitted to a film guide and a pressing rotating member as a pressing means is driven to move the film to follow up the pressing rotating member. The latter pressing rotating member drive system is often used because of an advantage that component parts can be fewer.
In the heating assembly in which the heating means and the press roller are face to face provided to form the pressure contact nip as a heating target material heating zone and the heating target material is treated by heat and pressure as in the heat-fixing assembly of the heating roller system or film heating system described above, in order to make the assembly high-speed and its waiting time shorter the press roller may be formed of an elastic material and the pressure contact nip formed between the roller and the heating means on account of the former""s elastic deformation may be made wider so that the time for providing a sufficient quantity of heat to the heating target material can be ensured to improve efficiency for imparting heat to the heating target material. However, only making the pressure contact nip merely wider makes the heating assembly itself larger and at the same time results in an increase in power consumption. Accordingly, the assembly must further be improved in thermal efficiency in order to make the assembly small-sized and to achieve cost reduction and low power consumption.
From the viewpoint of an improvement in thermal efficiency of the heating assembly, the quantity of heat taken away from the heating means to the pressing means side is not negligible. Accordingly, in order to make the assembly high-speed and achieve its low power consumption, it is desired to make the pressing means have a low heat capacity. As a means for making such a pressing means have a low heat capacity, it is known, as disclosed in, e.g., Japanese Patent Application Laid-Open No. 9-114281, that a pressing rotating member having superior heat insulation properties can be produced in a good mass productivity by incorporating an elastic layer of the pressing means press roller with a hollow filler.
As the hollow filler, inorganic fillers containing air in the interior such as hollow silica, alumina, glass and glass fiber are used. However, when such inorganic type hollow fillers are used, the fillers are so hard as to make the press roller""s elastic layer hard, bringing about a problem that a great pressure must be applied in order to ensure a wide fixing nip.
In addition, in heating assemblies of electrophotographic image-forming apparatus, the assemblies have become more small-sized in recent years, and press rollers used therein have also become more small-sized. Making press rollers have a small diameter brings about a tendency of making the elastic layer have a low hardness in order to ensure the nip width at the time of fixing; the elastic layer being covered on the periphery of a press roller mandrel. For example, as disclosed in Japanese Patent Publication No. 4-77315, those making use of a foamed elastic material (sponge rubber) in the elastic layer are put into practical use in a large number. However, at the time a blowing agent mixed in silicone rubber is heated to cause blowing, its blowing pressure may break cell walls of silicone rubber to make some cells of the resultant foam uncovered to the surface, or make very thin the cell walls that isolate the foam cells from the atmosphere, to form virtual concavities. Also, in the case when silicone rubber is blown in a mold, the blowing pressure extends in irregular directions, and hence irregular blowing stress stands occurred in the rubber. Thus, once the silicone rubber is taken out of the mold after blowing, such irregular stress is set free to cause irregularity or unevenness at the rubber surface.
If a silicone rubber roller comprised of such a foam is used as the press roller, the fused toner having offset-adhered to the heating roller or heating film is transferred to cause contamination of the press roller.
Where a sponge elastic member as a foamed elastic member comprising a mandrel and a foamed silicone rubber provided thereon is formed and a heat-resistant release layer of a fluorine resin such as PFA or PTFE is formed on its periphery by coating, its coating agent may enter the uncovered cells or concavities of the foam to make it difficult to form a release layer having a smooth surface and a uniform thickness. Also, where the release layer is formed by covering the sponge elastic member with a fluorine resin tube, there has been a problem that the fluorine resin tube may become uneven after the shape of uncovered cells of the foam when it stands under pressure and hence, when paper is fed, the press roller is contaminated because of an offset slight enough to be invisible and a toner present on the back of the paper.
The press roller used in heat-fixing assemblies is required to have properties that its hardness and thermal conductivity do not change because of any heat history repeated over a long period of time. This is because the nip width changes with a change in hardness and also the fixing efficiency decreases with an increase in thermal conductivity.
As one of methods of producing the sponge rubber, a method is also known in which resin microballoons are utilized. As an example thereof, as disclosed in Japanese Patent Applications Laid-Open No. 8-12888 and No. 5-209080, unexpanded microballoons are mixed in rubber, followed by heating to cause the resin microballoons to expand and harden simultaneously.
As another method, for the purpose of solving a problem (non-uniformity of cells) in the above method, proposed as a method of producing the sponge rubber is a method in which resin microballoons having previously been expanded are mixed in a liquid compound and a cross-linked rubber molded product is obtained at a temperature not higher than the resin melting temperature and also proposed is a transfer drum produced by such a method (Japanese Patent Application Laid-Open No. 10-060151).
Expanded resin microballoons are used as fillers in various coating materials and plastic materials.
Since, however, they tend to fly in all directions (scatter), a method is proposed to prevent scattering. For example, Japanese Patent No. 02822142 discloses a method in which unexpanded microballoons and a wetting agent (plasticizer) are mixed at a temperature not higher than the expansion start temperature of the unexpanded resin microballoons and thereafter the mixture obtained is heated to a temperature near to the expansion start temperature of the unexpanded resin microballoons to obtain expanded resin microballoons. Japanese Patent Application Laid-Open No. 6-240040 also discloses a method of producing microballoons that may less scatter and have superior handling properties; the microballoons being characterized in that fine inorganic material particles are fastened through a binder resin to the surfaces of microballoons formed by heat-expanding thermoplastic resin microcapsules containing a low-boiling organic solvent.
However, in the method of producing a silicone rubber sponge compounded with the resin microballoons having previously been expanded, the expanded resin microballoons have a very low specific gravity, and have a problem that they must be stored in a very bulky form and besides can be mixed in silicone rubber materials with great difficulty. In the prior art method in which unexpanded microballoons and a wetting agent (plasticizer) are mixed at a temperature not higher than the expansion start temperature of the unexpanded resin microballoons and thereafter the mixture obtained is heated to a temperature near to the expansion start temperature of the unexpanded resin microballoons to obtain expanded resin microballoons, the wetting agent (plasticizer) is exemplified by phthalate type plasticizers, aliphatic dibasic acid ester type plasticizers and epoxy type plasticizers. These, however, have a poor compatibility with liquid silicone, and may cause a problem on storage stability, such as separation, when the expanded resin microballoons are mixed with the liquid silicone.
In the case of the microballoons to the surfaces of which fine inorganic material particles are fastened through a binder resin and which may less scatter and have superior handling properties, too, no sufficient heat insulation properties can be achieved in some cases.
Under such circumstances, it is sought to provide a method which makes use of neither the wetting agent nor the fine inorganic material particles when expanded resin microballoons are used as a filler.
Accordingly, a first object of the present invention is to provide a heating assembly making use of a press roller which has a low thermal conductivity, may hardly take heat away from the heating means, has a low surface hardness, and can make the fixing nip wider, and an image-forming apparatus having such a heating assembly as a heat-fixing assembly.
A second object of the present invention is to provide a heating assembly that may hardly cause contamination of its press roller with toner.
A third object of the present invention is to provide a process for producing a silicone rubber sponge and a silicone rubber sponge roller by using resin microballoons, in which the resin microballoons have been prevented from scattering without affecting the thermal conductivity (heat insulation properties) of the silicone rubber produced.
A fourth object of the present invention is to provide a process for producing a roller that does not change in hardness and thermal conductivity even where it has undergone heat history as a press roller used in the heat-fixing assembly.
First, the present invention is a heating assembly comprising a heating means for heating a sheetlike heating target material and a press roller disposed face to face with the heating means; the heating target material being guided to a pressure contact nip formed between the heating means and the press roller so as to be held and transported between them to heat the heating target material; wherein;
the press roller has an elastic layer incorporated dispersedly with voids formed by resin microballoons.
The voids incorporated dispersedly in the elastic layer of the press roller used in the heating assembly of the present invention are formed by resin microballoons. The resin microballoons are an organic filler, and they are softer than inorganic fillers and do not make the elastic layer excessively hard. Hence, the fixing nip (pressure contact nip) can be formed in a sufficient width upon application of light pressure. Also, since the resin microballoons are an organic filler, they have a lower thermal conductivity than inorganic fillers, and are advantageous in that a thermal conductivity of 0.146 W/mxc2x7K or lower which is desirable for the elastic layer can be achieved.
The resin microballoons are also microballoons whose shells are formed of a resin and in the interiors of which a gas is enclosed. Hence, the resin microballoons neither forms any cells uncovered to the elastic layer surface nor forms any concavities at the elastic layer surface. Also, even when the elastic layer containing such resin microballoons dispersedly is formed by mixing unexpanded resin microballoons internally holding a volatile substance, with an elastic material and thereafter heat-expanding the unexpanded resin microballoons, the pressure of expansion that is ascribable to the volatile substance is checked by the shells and hence neither uncovered cells nor concavities are formed at the elastic layer surface. Thus, a press roller that can be free of contamination with toner can be provided.
Second, the present invention is a process for producing a silicone rubber sponge, comprising the steps of heat-expanding unexpanded resin microballoons wet-treated with a silicone oil, mixing the heat-expanded resin microballoons in a liquid silicone rubber material, and heat-curing the liquid silicone rubber.
In this silicone rubber sponge production process of the present invention, the unexpanded resin microballoons are wetted with a silicone oil and thereafter heat-expanded at an appropriate temperature, so that the surfaces of expanded resin microballoons stand covered with the silicone oil in a very small quantity, and hence the balloons readily adhere to one another and can be prevented from scattering. Also, since the silicone oil is a material equivalent to the silicone rubber, it does not substantially affect the thermal conductivity of the silicone rubber sponge produced.
Third, the present invention is a process for producing a roller, comprising the steps of heat-expanding unexpanded resin microballoons, mixing the heat-expanded resin microballoons in a liquid silicone rubber material, heating the mixture on a mandrel to cure the liquid silicone rubber, and, after heat-curing the liquid silicone rubber material, heating the resin microballoons at a temperature not lower than the expansion start temperature of the resin microballoons to break the shape of microballoons the shell resin of the resin microballoons provides. The process may preferably further comprise, after breaking the shape of the resin microballoons, the step of forming a release layer on the roller surface. The roller thus produced is especially effective as a press roller making use of resin microballoons not having a sufficient high-temperature heat resistance, i.e., as a press roller of a heat-fixing assembly comprising a heating means for heating a recording medium holding thereon an unfixed image, to fix the unfixed image, and a press roller disposed face to face with the heating means and brought into pressure contact with the heating means to form a pressure contact nip between them.
More specifically, where a powder consisting of particles making use of a thermoplastic resin in shells and holding a volatile substance internally is employed as the unexpanded resin microballoons, an elastic layer is formed in which heat-expanded resin microballoons are dispersed in silicone rubber sponge as they are.
In such an elastic layer, the thermoplastic resin is harder than the silicone rubber and hence makes the elastic layer hard. Also, when used as the press roller of the heat-fixing assembly, the roller undergoes heat history, so that the shells formed of the thermoplastic resin may break or undergo thermal decomposition or carbonization to lose the hardness attributable to the presence of the shells, resulting in a decrease in roller hardness or an increase in thermal conductivity to cause variations in fixing performance. Such a break of shells tends to be caused by what is called temperature rise at non-paper-feed areas when the roller is used as a press roller for heat fixing. That is, the resin microballoons undergo thermal damage at non-paper-feed areas to cause a local decrease in roller hardness to bring about a problem on transport performance in some cases. This is because, where small-sized sheets of paper are continuously passed, the non-paper-feed region of the press roller is continuously directly heated by a fixing member, and hence, even when the paper-feed region on the press roller surface is maintained at 150xc2x0 C. or below, the surface temperature of the non-paper-feed region may reach about 250xc2x0 C.
Accordingly, in the above roller production process of the present invention, the shells which provide the shape of resin microballoons are broken, whereby such problems can be solved. Also, the shells which provide the shape of resin microballoons may be broken at any stage, i.e., before or after the release layer is formed or at the same time the release layer is formed. In the case when the shells which provide the shape of resin microballoons are broken after the release layer is formed, the gas component generated with the break of resin is enclosed and there is a possibility of deteriorating the silicone rubber depending on the type of the resin to be broken. Hence, the shells which provide the shape of resin microballoons may most preferably be broken before the release layer is formed.